manifest: Introduce sync-j-max attribute to cap sync jobs

Add a way for manifest owners to limit how many sync jobs run in
parallel.

Bug: 481100878
Change-Id: Ia6cbe02cbc83c9e414b53b8d14fe5e7e1b802505
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/548963
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

man/repo-manifest.1

@@ -1,5 +1,5 @@
 .\" DO NOT MODIFY THIS FILE!  It was generated by help2man.
-.TH REPO "1" "December 2025" "repo manifest" "Repo Manual"
+.TH REPO "1" "February 2026" "repo manifest" "Repo Manual"
 .SH NAME
 repo \- repo manifest - manual page for repo manifest
 .SH SYNOPSIS
@@ -131,6 +131,7 @@ include*)>
 <!ATTLIST default dest\-branch CDATA #IMPLIED>
 <!ATTLIST default upstream    CDATA #IMPLIED>
 <!ATTLIST default sync\-j      CDATA #IMPLIED>
+<!ATTLIST default sync\-j\-max  CDATA #IMPLIED>
 <!ATTLIST default sync\-c      CDATA #IMPLIED>
 <!ATTLIST default sync\-s      CDATA #IMPLIED>
 <!ATTLIST default sync\-tags   CDATA #IMPLIED>
@@ -309,7 +310,9 @@ when syncing a revision locked manifest in \fB\-c\fR mode to avoid having to syn
 entire ref space. Project elements not setting their own `upstream` will inherit
 this value.
 .PP
-Attribute `sync\-j`: Number of parallel jobs to use when synching.
+Attribute `sync\-j`: Number of parallel jobs to use when syncing.
+.PP
+Attribute `sync\-j\-max`: Maximum number of parallel jobs to use when syncing.
 .PP
 Attribute `sync\-c`: Set to true to only sync the given Git branch (specified in
 the `revision` attribute) rather than the whole ref space. Project elements